A double-blind crossover study evaluated the clinical response of hyperactive boys to Sinemet (maximum dose, 60-600 mg). The behavioral and cognitive responses were qualitatively similar to that seen with amphetamine but the effects were much weaker; clinically it does not appear the Sinemet will be useful in the treatment of hyperkinetic children. Thoretically, Sinemet increased urinary MHPG, an index of central norepinephrine metabolism and therefore the mild improvement seen does not support a specific dopamine hypothesis of therapeutic action of stimulants in these children.